Preferable nonfibrous porous layers, in the above blood cell-separating layer composed of a fibrous porous layer and a nonfibrous porous layer joined by partial adhesion, are blushed polymer layers composed of a cellulose ester, such as cellulose acetate, cellulose acetate/butyrate or cellulose nitrate, disclosed in U.S. Pat. Nos. 3,992,158 or 1,421,341.